Pressure-compensated spool valves have been previously made using close-fitting pistons. That technique requires tight tolerances on the spool and the valve bore. Viscous and contact friction exist between the spool and bore, and their presence limits the extent to which such as a valve can be successfully pulse-width-modulated in response to the application of a pulse-width-modulated energizing signal to the solenoid.
The present invention provides a solution to this problem by the use of one or two flexible diaphragms to support the mechanism by means of which motion of the solenoid's armature is transmitted to the valve element that controls the fluid communication between the valve's source, control, and drain ports. The mechanism no longer requires the close fit of a spool to a bore. Pilot pressures from selected ones of said ports are communicated by pilot passages to opposite sides of each diaphragm used.
One embodiment of valve constructed in accordance with the inventive principles comprises a single diaphragm whose opposite sides are piloted to the control and drain ports to provide pressure-compensation with respect to the control/drain port pressure differential. In another embodiment, twin diaphragms pressure-compensate the valve with respect to source/control port pressure differential and control/drain port pressure differential. In the latter embodiment, one side of one diaphragm is piloted to source port pressure, and the opposite side of the same diaphragm is piloted to control port pressure; one side of the second diaphragm is piloted to control port pressure while its opposite side is piloted to drain port pressure.
The essentially frictionless nature of the operation of a diaphragm, particularly a rolling bellows type diaphragm, allows the valve to have a sufficiently fast response for use in pulse-width modulated pressure control applications. The pressure-compensation forces applied to the mechanism counter hydraulic forces acting on the valve element so that the transmittal of hydraulically-induced forces to the solenoid can be controlled. The relationship of diaphragm area to valve seat area determines the nature of the pressure-compensation. By properly relating the areas in a particular way, it is possible to essentially eliminate the transmission of any hydraulically-induced forces to the solenoid. By properly relating the areas in any one of other particular ways, it is possible to impart any one of a number of different hydraulically-induced bias force characteristics. In any particular valve, the sizing of the diaphragms and valve seat areas is conducted in accordance with conventional principles of hydraulic design. Hence, the pressure-compensating feature that results from the practice of principles of the present invention can be embodied in any of a number of different ways to attain any particular one of a number of different pressure-compensation characteristics for a three-way valve.
It should be pointed out that the use of pressure-compensating diaphragms in valves is not novel per se. U.S. Pat. Nos. 3,606,241 and 3,985,333 illustrate this. The present invention is however a novel application of pressure-compensating diaphragms to a three-way valve through the use of pilot passages. The improved responsiveness of a three-way valve embodying principles of the present invention and the ability to design such a valve to attain a desired pressure-compensation characteristic are especially significant attributes of the invention. As a consequence, the invention adapts a three-way solenoid valve for a wider range of pulse-width-modulated uses, including an electrically controlled transmission valve in an automotive vehicle transmission.